The present invention relates to a contour coding method for object oriented video coding. The present invention also relates to a contour motion information transmission method.
A system requiring low bit rate data transmission compresses image information at a high ratio in order to use motion image. For doing so, region based motion image coding techniques for dividing an image into regions and performing motion estimation and coding operations by regions, have been proposed instead of a conventional block-based image coding technique. The region-based motion image encoder largely relies on an algorithm for encoding region division information in view of its performance. Therefore, techniques for coding contours efficiently are needed.
Referring to FIG. 1 showing region-based motion compensation, assuming that the motion compensation is performed using the object in the previous frame, motion compensated errors will occur around an object along the motion trajectory, due to the inaccuracy in the motion estimation or the deformation of the object. If the region with large motion compensation errors is defined as the motion compensation failure region, the motion compensation failure region (hereinafter, called "MF region") is considered as the mismatched region between a current object and the motion compensation object. In FIG. 1, the MF regions are indicated as shading portions neighboring the current object and the contours surrounding the current object are denoted by thick lines.
Generally, an encoder and a decoder involving a region-based motion image coding know information with respect to the contours of an object in a previous frame in advance. Thus, the contours in the current frame can be effectively encoded using motion compensation. When the motion compensation is used, only contour pixels corresponding to the MF region among the contour pixels contained in the contours of the object in the current frame are required for transmission, since the others can be estimated from the contours of the motion compensation object in the previous frame.
A reference 1! entitled "Contour simplification and motion compensated coding", published in Special Issue of Signal Processing: Image Communication on Coding Techniques for Very Low Bit-rate Video, Vol. 7, No. 4-6, pp. 279-296, November 1995, by C. Gu and M. Kunt, discloses a predictive contour coding technique for video coding using temporal correlation. According to the reference 1!, after motion-compensation of each object, the contour pixels in the MF regions are encoded and transmitted by the conventional chain coding technique.
However, since the MF regions are generally distributed in a sparse and isolated manner, each isolated contour requires a number of overheads to represent it with chain codes. Further, since the motion compensation is performed on an object basis, even small local motion behavior regions within the object are determined as MF regions. Therefore, inter frame correlation of the contours cannot be exploited.